Past industrial activities have contaminated sediment in many streams, rivers, lakes, and harbors. The contaminated sediment requires remediation to mitigate its potential impact on ecological receptors, human health, or environmental media. An overview of sediment remediation options is provided below.
In-situ Capping—In-situ capping isolates contaminated sediment from the surrounding surface water body or ecological receptors by placing a protective cover over the contaminated sediment area.
In-situ Treatment—treatment refers to treatment of the contaminated sediment at its current location without removal. The treatment methods include biological, chemical, and physical processes.
Removal—Removal is a necessary step for other remediation methods such as ex-situ treatment, off-site disposal or on-site disposal. The most common removal method is dredging. Excavation is also used if the sediment is under a shallow water body that may be drained temporarily using a simple and economical surface water barrier.
Ex-situ Treatment—In this approach, contaminated sediment is removed from its current location and treated. Ex-situ treatment methods include bioremediation, chemical treatment, soil washing, solidification/stabilization and others.
Off-site Disposal—Even after ex-situ treatment, the quality of treated sediment may not fully meet all regulatory requirements. In this case, the treated sediment is taken to an off-site disposal facility (sanitary, industrial or hazardous waste landfill) for safe disposal.
On-site Disposal—Contaminated sediment may be removed and contained, with or without treatment, in an engineered disposal facility built at the project site solely for disposal of the target sediment. The disposal facility filled with sediment is closed as a landfill. Therefore, sediment dewatering is essential. Two common dewatering methods are mechanical dewatering and geotube dewatering.
In mechanical dewatering, dredged sediment is pumped to a mechanical dewatering unit (e.g., a centrifuge, a belt press, or a filter press), dewatered, and cake is placed in the disposal facility. Often, the cake requires solidification/stabilization as cake from mechanical dewatering cannot support earthwork equipment used for disposal work.
Geotube dewatering uses geotubes for dewatering. Geotubes are large filter bags made of geotextile. Dredged sediment is pumped into a geotube and water is allowed to drain, leaving solids in the geotube. After the geotube is filled with dredged sediment, the sediment is allowed to drain for some time. When the geotube collapses as water is drained, more dredged sediment is pumped into the geotube. After cycles of filling and draining, the geotube is filled with “drained” sediment. The drained sediment may be dewatered further, if desired, by evaporative drying for several weeks. The dewatered sediment may be taken off site for disposal. For on-site disposal, geotubes may be deployed within the disposal pond before they are filled.
Contained Aquatic Disposal—Contained aquatic disposal is underwater disposal and capping of dredged sediment in natural depressions, excavated pits or bermed areas at the bottom of water bodies. This method is often used for disposal of the sediment dredged from harbors and urban waterways where on-site disposal is not feasible due to limited land area. The disposal sites are selected from areas with a sufficient water depth (to avoid interruption of navigation) and low water energy (to avoid erosional loss of contained sediment).
Consolidation refers to a process of soft clayey soils subject to a load undergoing volume reduction and strength gain as a result of water being squeezed out of the loaded soil volume. As clayey soils do not allow water to flow out easily due to its very low hydraulic conductivity, drainage pathways are provided in the soil volume to accelerate consolidation. The most common way of providing drainage pathways is to insert wick drains vertically into the clay layer with a typical spacing of about 1.5 m. A wick drain is a long strip about 0.5 cm thick and 10 cm wide and consists of a plastic core wrapped with geotextile filter. Wick drains facilitate flow of water from soft clayey soils to the ground surface.
Accelerated consolidation with wick drains has been used for numerous construction projects on soft clayey soils. However, it has not been used often for dewatering of dredged sediment in environmental remediation due to the inherent limitations described below. As consolidation is a method of stabilizing a full layer of soft soil, it is applicable to dredged sediment only after the disposal operation is completed. However, consolidation dewatering after filling a disposal pond with dredged sediment is not practical for the reasons described below.
To illustrate the point, suppose that consolidation dewatering is attempted for disposal of dredged sediment. Dredged sediment typically contains less than 10% solids by weight as it is pumped as a slurry. After settling in the disposal pond, its typical solids content is around 35% by weight, equivalent to 17% solids and 83% water by volume. As this is too soft to place a final cover for closure, the dredged sediment requires dewatering, in this case by consolidation. The pond surface must be stabilized first by draining and natural drying to allow equipment access. This step takes a very long time. The subsequent steps of consolidation work include covering the surface with a geotextile, spreading 0.5 to 1.0 m of sand (top blanket drain) over the geotextile, installing vertical wick drains into the soft sediment with an installation rig working on the top blanket drain, and loading with thick surcharge fill. As this fill cannot be placed in one step on the very soft sediment, it must be placed in small lifts, allowing time for consolidation and strength gain before applying the next lift. Thus, this loading step also takes a long time. A large settlement, typically about 50% of the initial sediment thickness, occurs as a result of consolidation. The final step of pond closure would be surface grading and final cover installation. Surface grading requires the surcharge fill equivalent to the total consolidation settlement to remain in the pond.
The steps described highlight three major problems in consolidation dewatering for on-site disposal of dredged sediment. First, these steps take very long, particularly in stabilizing the surface for equipment access and in applying the surcharge load in several lifts. Second, the capacity of the disposal pond is wasted by the fill equivalent to about 50% of the pond capacity. Third, the above two reasons make consolidation dewatering costly and impractical. For these reasons, consolidation dewatering is not viable for disposal of dredged sediment in environmental remediation, unless technical improvements are made. The above-described problems can be overcome if the sediment in the disposal pond is consolidated while dredged sediment is being discharged into the pond. Thus, it would be desirable to devise a method of consolidation dewatering concurrent with discharge of the dredged sediment into the disposal pond.
In achieving the goal stated above, vacuum loading will play a key role. Vacuum has been often utilized as a means of loading for consolidation projects. In this method, the ground surface is covered with an impermeable membrane and vacuum is applied to the underside of the membrane. This creates an effect of atmospheric pressure working as a load. Although vacuum consolidation offers some advantages, it is often troublesome due to incomplete seals along the edge of the membrane and its cost is significant. A Dutch firm COFRA (see COFRA webpage) practices a vacuum loading method that does not require membrane sealing by connecting the top of vertical wick drains with sealed vacuum lines within the soft clay layer, which is almost impermeable. The present invention intends to extend vacuum consolidation application to horizontal drains using self-sealing properties of fluid earthen medium which is the target for consolidation.
The Corps of Engineers performed a research project evaluating ways of stabilizing dredge spoils from navigation dredging and demonstrated that vacuum underdrainage is an effective way of stabilizing dredge spoil (Hammer, 1981). In this method, a layer of bottom blanket drain is installed in the disposal facility, dredge spoil is discharged, and a vacuum is applied to the bottom blanket drain.
In-situ capping refers to the isolation of contaminated sediment from the water column above by covering or capping the contaminated sediment area with clean barrier materials. The primary functions of in-situ capping include: physical isolation of the contaminated sediment from the benthic environment; stabilization of contaminated sediment to prevent re-suspension and transport to other sites; and/or, reduction of the flux of dissolved contaminants from the contaminated sediment into the water column above (EPA, 2005).
To perform these primary functions, in-situ caps may be designed with various materials such as sand, gravel, or a mixture of earthen materials. For better performance and protection, in-situ caps are also designed with multi layer components. Materials used for multilayer caps include various earthen materials, geotextiles, low permeability liners, and reactive layers for treatment or adsorptive sequestration of contaminants. An armor rock layer often tops the in-situ cap to maintain the stability of the cap against turbulent flows, wave actions or ice jams.
In-situ capping of contaminated sediment s well known. Examples of in-situ capping in prior art are shown in FIGS. 6a, 6b and 6c. FIG. 6a shows a simple form of in-situ capping, wherein a sand cap (often a foot or thicker) is placed over the surface area of contaminated sediment. FIG. 6b shows another form of in-situ capping wherein a sand cap is placed over the surface area of contaminated sediment and the sand cap is protected by a layer (often a foot or thicker) of armor rock. In FIG. 6b, geotextile is used between sand cap and armor rock and between contaminated sediment and sand cap to prevent mixing of the two different materials. FIG. 6c shows an example of multilayer cap that includes a sand cap placed over the surface area of contaminated sediment, a layer of armor rock to protect the sand cap, a barrier layer (a low permeability liner) to minimize the upward flux of contaminated pore water from the sediment, and a reactive layer to treat or adsorb contaminants passing across the barrier.
A known problem with in-situ, submerged capping is loss of water depth. Installation of an in-situ cap always reduces the water depth. At many sites, in-situ capping is not allowed due to institutional restrictions prohibiting any loss of water depth. Examples of such institutional restrictions include no loss of channel flow capacity, no loss of flood storage capacity, no loss of navigation depth, no loss of preferred habitat, and no loss of wetlands.
In-situ capping in general cannot overcome these restrictions because a loss of water depth is inevitable when cap materials are placed over the existing sediment as shown in prior art FIGS. 7a and 7b. A section of a submersed, contaminated-sediment site is schematically illustrated in FIG. 7a. The site includes a body of water, such as a lake or river, having a top water surface 101, water column 102, and water depth DW1. The water body sits above or flows over a contaminated sediment layer 105 having a top surface 104, and an initial thickness TCS1. The contaminated sediment layer 105 overlies an uncontaminated sediment layer 107 having an initial thickness TUS1. The uncontaminated sediment layer overlies firm soil or a bedrock surface 109.
FIGS. 7a and 7b show the change in the water depth from DW1 to DW2 after installation of an in-situ cap 111 with its thickness TCP. The loss of water depth is generally equivalent to TCP. Due to the load of the in-situ cap 111, the contaminated sediment 105 and uncontaminated sediment 107 may be compressed and settle a little but as its magnitude is very small, it is not shown in FIG. 7b for clarity. As a result, prior art in-situ capping methods cannot be used at contamination sites that prohibit any loss of water depth. To avoid the costly alternative of dredging and off-site disposal of contaminated sediment, it would be desirable to provide a method of in-situ capping that does not cause a loss of water depth.